Semiconductor laser devices such as ridge waveguide lasers and laser amplifiers are used in many communications systems. Incremental refinements in their fabrication and packaging have resulted in a class of devices that have acceptable performance characteristics and a well-understood long-term behavior. Moreover, the ridge waveguide structures are less complex to fabricate and provide excellent yields as compared to more complex architectures based on buried heterostructures, for example.
In most applications, maximizing the laser's or amplifier's useful operating power is a primary design criteria. In long distance communication applications, the power output from the device dictates the distance to the next repeater stage, and the number of stages in a given link is a major cost factor in the link's initial cost and subsequent maintenance.
The useful operating power of laser devices is limited in many applications by a "kink" in the power versus current dependence above the lasing threshold, and weakly-guided semiconductor devices, such as ridge waveguide lasers, are particularly susceptible to these kinks. Kink definitions vary greatly but typically correspond to deviations of approximately 20% from a linear dependence above the threshold.
A number of different theories have been proposed to explain the kink in the power vs. current dependence. The theories agree insofar as there appears to be a shift of the eigenmode space at the higher currents that affects the total optical output and/or how the output is coupled into a fiber transmission media.
Notwithstanding the theoretical uncertainty, experimentation has demonstrated that the kink power for a given laser device is strongly dependent on its resonant cavity characteristics, e.g., cavity dimensions and refractive indices and their profile. For example, in the case of a weakly-guided gallium arsenide ridge waveguide devices, the characteristics of the resonant cavity are dictated in part by cladding layer parameters. Unfortunately, these cladding layer parameters and the fabrication processes used to define the parameters are difficult to control with the accuracy required for a single resonator design to be optimum for all wafers and all devices.
To compensate for parameters that can not be controlled with high accuracy, fabrication of ridge waveguide devices must be optimized for each base wafer to achieve acceptable kink power performance. Variations in the wafers are measured, and the acquired information is used to individualize the wafer's processing.